Glass-rubber transition frequency effects

In amorphous polymers the glass transition and the relaxation behavior associated with it are very sensitive to the addition of small amounts of diluents. As the diluent is added, the relaxation is shifted to a lower temperature at constant frequency or higher frequency at constant temperature. The usual explanation is that since the diluent molecules are small and mobile, they act to effectively increase the available free volume for segmental motion and hence speed it up. Similar plasticizing effects on the glass-rubber transition in semicrystalline polymers are observed. One such example is the case of poly(vinyl alcohol), which is water-soluble (Takayanagi 1965). Other examples of semicrystalline polymers where the effects of moisture are observed are aliphatic polyamides such as Nylon 6-6 (Starkweather 1980), Nylon 6-10 (Boyd 1959 Woodward et al. I960), and Nylon 12 (Varlet et al. 1990). 

The rabber modulus increases with an increasing volume fraction of Aerosil. The modulus increase can be caused by the elastomer-filler and filler-filler interactions and by an increase of effective filler content. A very sharp peak for the tanZ is observed at 163 K for an unfilled crosslinked sample. This maximum corresponds to the glass transition of the rubber. Furthermore, it is observed that the Tg of the rubber does not change in the presence of filler. However, the second maximum of to 5 can be seen in the vicinity of 200 K for filled samples. The intensity of this maximum becomes more pronounced with increasing Aerosil content. This observation is in agreement with the results of the h and Ty relaxation study, as demonstrated in Fig. 4a and 6, respectively. Therefore, it seems reasonable to assign the maximum for at 200 K to the motion of adsorbed chain units. This maximum is observed at a lower temperature than the H and T, minimum for the adsorbed chain units (at about 280 K) due to difference in frequency of these methods 1.6 Hz and 46-90 MHz, respectively. 

There have been very few studies reported on the viscoelastic properties of rubber-resin pressure sensitive adhesive systems. In 1973, M. Sherriff and co-workers (1) reported on the effect of adding poly (j3-pinene) resin to natural rubber. Based on a G master curve, they showed that the resin shifted the entry to the transition zone to a lower frequency and reduced the modulus in the rubbery plateau. G. Kraus and K.W. Rollman (2) reported in 1977 on their study of resins blended with styrene-isoprene-styrene block copolymers. They showed that the addition of a resin increased the glass transition temperature of the rubbery mid-block and decreased the plateau modulus. Accordingly, a satisfactory tackifying resin should produce these changes. 

If the glass transition temperature increases at about 6 to 7°C per decade of frequency, the effective glass transition temperature of the rubbery phase is increased about 60°C above values measured at low frequencies, about 10 Hz. This calculation, while grossly oversimplified, suggests that the Tg of the elastomer phase must be about 60°C below the test temperature, which correlates well with the experimental evidence (see Table 11.4) (18). Thus, if the impact experiments are done at about 20°C, the glass transition of the rubber must be below about -40°C in order to attain significant improvement in impact resistance (21). 

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